1. Field of the Invention
This invention relates to switched reluctance motors, and, more particularly, to a method and a circuit for controlling the current in each motor phase of a switched reluctance motor between two predetermined current levels.
2. Disclosure of Related Art
A conventional switched reluctance motor (SRM) includes a stator having a plurality of pairs of diametrically opposed stator poles and a rotor having a plurality of pairs of diametrically opposed rotor poles. Windings or coils are typically disposed about the stator poles and the windings around any two diametrically opposed stator poles may be connected in series or in parallel to define one motor phase of the multiphase SRM. The windings associated with a motor phase may be referred to as a phase coil. By generating current through the phase coil, magnetic fields are established about the stator poles and a torque is produced that attracts a pair of rotor poles into alignment with the stator poles. The current in the phase coils is generated in a predetermined sequence in order to produce a constant torque on the rotor. The period during which current is provided to the phase coil--and the rotor poles are brought into alignment with the stator poles--is known as the "active stage" of the motor phase. At a certain point--either as the rotor poles become aligned with the stator poles or at some point prior thereto--it becomes desirable to commutate the current in the phase coil to prevent a negative or braking torque from acting on the rotor poles. Once this "commutation point" is reached, current is no longer generated in the phase coil and the current is allowed to dissipate from the phase coil. The period during which current is allowed to dissipate from the phase coil is known as the "inactive stage" of the motor phase.
SRMs typically employ pulse width modulation (PWM) or current chopping to control the current level in each phase coil during the active stage of the motor phase. Conventional circuits and methods for performing PWM or current chopping have often incorporated microprocessors. The use of microprocessors is disadvantageous, however, because it increases the cost of the motor and limits the bandwidth of electrical signals within the control system for the motor.
There is thus a need for an apparatus and method for controlling a switched reluctance motor that will minimize or eliminate one or more of the above-mentioned deficiencies.